1. Field of the Invention:
The present invention relates to a video signal hard copying apparatus capable of automatically setting a sampling clock for a video signal, and of being connected to varieties of computers by automatically adjusting the characteristics of unknown signals such as levels, scanning systems and logics.
2. Description of the Prior Art:
Recent development of small computers requires graphical representation with the need of a color hard copying apparatus to exhibit and record any graphical data. Many makers manufacture those small computers of all sorts. It is accordingly needed for the color hard copying apparatus additionally provided in those computers not to be affected by specifications and characteristics of those computers. Thereupon, the color hard copying apparatus electronically processes a video signal composed of three primary colors displayed on a CRT screen, and prints out the signal as a color image. Such a video signal however occupies a wide frequency band of for example 100 MHz. Additionally, the video signal is not necessarily standardized in its signal level and in its transmission signal format for a synchronizing signal. Skilled artissans must therefore operate the color hard copying apparatus for installation, phase and level adjustment, and selection of synchronizing signal lines, etc. To solve this difficulty, there is known an automatic color hard copying apparatus, which is capable of automatically setting video signal levels, phases of R, G, B signals, and data regions to be incorporated therein. However, video hard copying apparatuses of this type suffer from the following problems. The automatic setting function is limited for example to phase correction for high frequency video signals or to detection of white and black levels of the same, etc, only for the purpose of avoiding variations of the operation in the same computer. That is, the apparatus is incapable of connection to varieties of computers without limitation, requiring costly design and manufacture thereof for each computer.
Additionally, such a video hard copying apparatus is supplied with a sampling clock for sampling the video signal, from the outside or from a frequency multiplier having a phase-locked loop (PLL) provided therein. The phase-locked loop is well known and includes, as illustrated in FIG. 8, a phase comparator 1 for receiving at one input a horizontal synchronizing signal Sh transmitted from a video terminal at one input thereof and an output Sd from a frequency divider 2 described later at the other input, comparing phases of both signals Sh and Sd, and outputting a pulse signal Sp with its width corresponding to a difference between both phases compared as such to a low-pass filter 3. The low-pass filter 3 transmits DC voltage Sc extracted therethrough from the pulse signal Sp to a voltage-controlled oscillator 4 (hereinafter referred to as a VCO), which then outputs a pulse train So with a frequency corresponding to the DC voltage Sc input thereinto to the foregoing frequency divider 2. The frequency divider 2 divides the pulse train So by a predetermined dividing ratio N and outputs the output Sd so divided. The prior PLL however has a problem of its being incapable of generating a sampling clock signal possessing the same frequency and phase as those of the horizontal synchronizing signal Sh unless being informed of accurate dividing ratio data. Such dividing ratio data is made clear with reference to a manufacturing drawing of the video terminal or to the details of the specification of the same. This is not accessible to ordinary users. It is therefore difficult to establish the same signal as the clock signal at the video terminal.